Reflection reducing coating

ABSTRACT

The invention relates to a reflection reducing coating on a substrate which is formed from an alternating change layer system each having a lower and higher refractive index. In particular, the invention can be advantageously employed on surfaces of substrates such as optical elements and spectacle glasses, in particular. According to the object most different substrates are to be coated, and during the deposition of a reflection reducing coating an unacceptable temperature rise of the respective substrate is not to occur. With this, a coating is deposited in which the sum of the layer thicknesses of layers each having a higher refractive index is ≦5% of the total layer thickness of the coating, and the layers of the material having a higher refractive index within the layer sequence of the change layer system are uniformly distributed.

[0001] The invention relates to a reflection reducing coating accordingto the preamble of claim 1. With the solution according to the inventionthe reflection of light impinging upon the surface of substrates can beeffectively and largely decreased, which is desirable for many cases ofapplication, in particular for many optical elements (lenses, windowsand prisms etc.) or opto-electronic elements, and for spectacle glassesas well. The coating can be particularly advantageously employed for thereflectance decrease in the visual spectral range.

[0002] For a plurality of applications, and spectacle glasses are to beexplicitly mentioned, it is also required to provide a respective“antireflex coating” having a high abrasion resistance.

[0003] In the past, synthetic material has also enforced as material foroptical elements and spectacle glasses, in particular due to themanufacturing cost and the lower density in comparison with glasses.

[0004] Although the synthetic material has equivalent opticalcharacteristics with respect to the glasses used until now, however, itprovides a Substantially lower scratch resistance such that with themechanical influences having an abrasive effect the surfaces becomedamaged and the optical performance is deteriorated.

[0005] For optical components, and in particular spectacle glassesabrasiveproof and reflection reducing surfaces are required which canonly be achieved by means of the respective coatings.

[0006] In accordance with the International Standard ISO 9211-02, forexample, such coatings have to meet a Sufficient wiping resistance alsoduring cleaning processes which are performed with cotton clothes orrubber.

[0007] In particular with the spectacle glasses hard layers having athickness of several micrometers, and thereupon an additional reflectionreducing coating are deposited.

[0008] Such hard layers can be manufactured by a layer of lacquer andthe subsequent vacuum evaporation of a reflection reducing coating suchas described by W. Köppen and E. Kampmeyer in DOZ 2(1995), pp. 22 to 26.

[0009] The generation of hard layers by means of plasma polymerizationoriginates from J. Bötschi, F. Thieboud, and is mentioned in DOZ 10(1992), pp. 26 to 27; and by D. Giessner in NOJ 5 (1995), pp. 62 to 64with respect to the vacuum evaporation of such hard layers, wherein thepublication last mentioned is a matter of organically modified quartzlayers. Subsequently, with depositing such hard layers again it isnecessary to deposit the reflection reducing cover layer by means ofwell-known CVD and PVD processes, respectively.

[0010] The same demands as with respect to other substrate materials aremade in particular on thermoplastic polymers such as poly(methylmethacrylate), polycarbonate and such other synthetic materials, whereinup to now accordingly suitable coatings are obtained in a wet-chemicalmanner or by means of vacuum evaporation, and a combination of suchcoating methods, respectively. Thus, a wet-chemical layer withwell-known coating methods is an additional time consuming and expensivemanufacturing step which is separated from the procedure step of formingthe reflection reducing layer. For precision optics having veryirregularly formed and greatly curved surfaces, and with small sizedoptical elements such methods are not suitable. With a reflectionreducing coating which has been deposited on a relatively thick, moreabrasiveproof hard layer an additional waviness of the spectralreflection of the total layer system occurs due to interference actionwith different refractive indices of the substrate material and hardlayer.

[0011] For broadband reflection reducing coatings which for examplecover the wavelength range of the visual light, two to six individuallayers are required. With the well-known change layer systems atemperature rise of the substrate temperatures can occur. Temperaturescan be achieved which are above of critical softening temperatures (e.g.80 to 110° C. with acrylates) of the substrate material. Then, thetemperature rise substantially occurs by the vaporization of the layermaterial having a high refractive index.

[0012] Therefore, it is an object of the invention to provide areflection reducing coating on a Substrate which can be deposited uponany substrates without occurring an unacceptable temperature rise duringdepositing.

[0013] In accordance with the invention this object is solved with acoating according to claim 1. Advantageous embodiments and improvementsof the invention can be achieved with the features mentioned in thesubordinate claims.

[0014] The reflection reducing coating according to the invention isformed from an alternating change layer system of different layermaterials each having a lower and a higher refractive index.

[0015] Then, the layer system will be configured such that the sum ofthe layer thicknesses of layers having a higher refractive index is ≦5%of the total layer thickness of the coating. These layers are largelyarranged in a uniformly distributed manner within the layer sequence ofthe change layer system. The uniform distribution and selection ofthicknesses of the very thin layers having a higher refractive indexoccurs under the consideration of a predetermined wavelength range ofthe light as well as the optical characteristics of the layer andsubstrate materials.

[0016] The substrate is optically translucent preferably in thewavelength range in which the reflection is to be prevented. Thetranslucence will be increased by means of the coating.

[0017] As a Substrate material the most different synthetic materials aswell such as e.g. polycarbonate and poly(methyl methacrylate) buttemperature sensitive crystal materials can also be employed in additionto the commonly used optical glasses, wherein the coating can bedeposited with methods for the substrate preparatory treatment and layerforming as are described in the DE 197 03 538 A1 and DE 197 52 889 C1.With the invention it is possible to exclude the undesired temperaturerise of the substrate material.

[0018] The oxides or fluorides of the IVth and Vth class B elements canbe preferably employed as layer materials for the individual layers ofthe change layer system having a higher refractive index. Such examplesare Ta₂O₅, ZrO₂, HfO₂, TiO₂ or indium tin oxide (ITO), as well.

[0019] For the layers of materials having a respective lower refractiveindex SiO₂ and MgF₂ can be advantageously employed, wherein inparticular SiO₂ has appropriate characteristics as a hard layer.Generally, the layer forming the boundary layer towards the air consistsof material having a low refractive index.

[0020] The change layer system reducing the reflection can be formedfrom layers of merely two materials having the respective refractiveindices. It is also possible to form such a change layer system from aplurality of such materials.

[0021] Such an change layer system can be matched to a predeterminablewavelength range of the light, wherein there are ways to providematching for the wavelength range of the visual light, light in thenear-infrared range and in the UV range as well.

[0022] For the change layer system at least five, preferably at leastnine individual layers have to be employed, however, wherein the numberof the layers can also substantially greater be selected.

[0023] The complete coating is allowed to have a total thickness between500 and 2500 nm, preferably between 750 to 2000 nm, in the visualspectral range.

[0024] If required, the coating according to the invention can bedeposited on a layer or coating already being present on the substratesurface.

[0025] When SiO₂, for example, as a hard layer component in a changelayer system is employed for a coating which is formed according to theinvention upon an optically translucent substrate then the layer systemforms a unit which simultaneously comprises a high abrasion resistanceand a high reflection reducing effect.

EXAMPLE 1

[0026] A coating for the visual spectral range in the wavelength rangebetween λ₁=420 nm and λ₂=670 nm, e.g. is allowed to have the followinglayer design which has been calculated with the constant refractiveindices of 1.5 for the substrate, 1.46 for the SiO₂ layers, 2.1 for theTa₂O₅ layers and 1.0 for air. The reflective performance in thewavelength range is diagrammatically shown in FIG. 1.

[0027] PMMA Substrate

[0028] 1^(st) layer 210 nm SiO₂

[0029] 2^(nd) layer 4 nm Ta₂O₅

[0030] 3^(rd) layer 251 nm SiO₂

[0031] 4^(th) layer 6 nm Ta₂O₅

[0032] 5^(th) layer 248 nm SiO₂

[0033] 6^(th) layer 9 nm Ta₂O₅

[0034] 7^(th) layer 237 nm SiO₂

[0035] 8^(th) layer 16 nm Ta₂O₅

[0036] 9^(th) layer 119 nm SiO₂

[0037] Air

[0038] The total thickness of the layer sequence is 1100 nm, wherein1065 nm is allotted to the SiO₂ layers which in the sum have an effectas a hard layer, and merely 35 nm is allotted to the Ta₂O₅ layersrequired for the additional antireflex effect.

[0039] By installing after the fifth layer of another layer pairs ofSiO₂ and Ta₂O₅ having a Similiar thickness the total layer thickness andthus the mechanical stability of the layer system can be increased. Byremoving the layers 4 and 5, and 4 to 7, respectively, the layer designcan be reduced without the antireflex effect getting lost.

[0040] The respective reflection performance of such layer systems isillustrated in the diagram shown in FIG. 2.

[0041] Coatings from SiO₂ and Ta₂O₅ layers on synthetic materials arepossible in the visual spectral range, e.g. with the following numbersof layers and total layer thicknesses:

[0042] 7 layers: appr. 850 nm

[0043] 9 layers: appr. 110 nm

[0044] 11 layers: appr. 1300 nm

[0045] 13 layers: appr. 1600 nm

[0046] 15 layers: appr. 1850 nm

[0047] 17 layers: appr. 2100 nm

EXAMPLE 2

[0048] In addition to coatings for the VIS range analogous layersequences are also possible for the NIR and UV ranges which areabrasiveproof as well as have an effect in a reflection reducing manner,and which can be deposited upon the sensitive substrate materialsmentioned. For the NIR wavelength range between λ₁=700 nm and λ₂=1110 nmthe following layer sequence having nine individual layers is possible.It has been calculated with constant refractive indices of 1.5 for thesubstrate, 1.46 for the SiO₂ layers, 2.1 for the Ta₂O₅ layers and 1.0for the air.

[0049] Synthetic Material Substrate

[0050] 1^(st) layer 349 nm SiO₂

[0051] 2^(nd) layer 6.5 nm Ta₂O₅

[0052] 3^(rd) layer 417 nm SiO₂

[0053] 4^(th) layer 10 nm Ta₂O₅

[0054] 5^(th) layer 412 nm SiO₂

[0055] 6^(th) layer 15 nm Ta₂O₅

[0056] 7^(th) layer 393.5 nm SiO₂

[0057] 8^(th) layer 26.5 nm Ta₂O₅

[0058] 9^(th) layer 197.5 nm SiO₂

[0059] Air

[0060] The total thickness of the layer sequence is 1827 nm, wherein1769 nm is allotted to the SiO₂ layers which in the sum have an effectas a hard layer, and merely 58 nm is allotted to the Ta₂O₅ layersrequired for the additional antireflex effect.

[0061] The reflection performance for this wavelength range has beenillustrated in the diagram of FIG. 3.

EXAMPLE 3

[0062] For the UV wavelength range between λ₁=290 nm and λ₂=470 nm thefollowing layer sequence including nine individual layers is possible.It has been calculated with constant refractive indices of 1.5 for thesubstrate, 1.46 for the SiO₂ layers, 2.1 for the Ta₂O₅ layers and 1.0for the air.

[0063] Synthetic Material Substrate

[0064] 1^(st) layer 147 nm SiO₂

[0065] 2^(nd) layer 3 nm Ta₂O₅

[0066] 3^(rd) layer 176 nm SiO₂

[0067] 4^(th) layer 4.5 nm Ta₂O₅

[0068] 5^(th) layer 174 nm SiO₂

[0069] 6^(th) layer 6.5 nm Ta₂O₅

[0070] 7^(th) layer 166 nm SiO₂

[0071] 8^(th) layer 11 nm Ta₂O₅

[0072] 9^(th) layer 83.5 nm SiO₂

[0073] Air

[0074]FIG. 4 shows the respective reflection performance in adiagrammatical manner.

[0075] The total thickness of the layer sequence is 771 nm, wherein746.5 nm is allotted to the SiO₂ layers which in the sum have an effectas a hard layer, and merely 25 nm is allotted to the Ta₂O₅ layersrequired for the additional antireflex effect.

[0076] The quality of the antireflex effect depends on the refractiveindex of the layer material having a low refractive index. In additionto the thus far mentioned SiO₂ having a refractive index of 1.46, e.g.MgF₂ having a refractive index of 1.38 is also possible as materialhaving a low refractive index. The layers having high refractive indexas well may be composed of another material e.g. of ZrO₂ having arefractive index of 2.0. Using material with a refractive index beingless tha SiO₂, e.g. with MgF₂, a value smaller than 0.5% can be achievedfor the mean residual reflection in the wavelength range between λ₁ andλ₂.

EXAMPLE 4

[0077] For a coating for the visual spectral range within the wavelengthrange between λ₁=420 nm and λ₂=670 nm using the materials MgF₂ and ZrO₂,e.g., the following layer design is possible. It has been calculatedwith constant refractive indices of 1.5 for the substrate, 1.38 for theMgF₂ layers, 2.0 for the ZrO₂ layers and 1.0 for the air. The reflectiveperformance is shown in FIG. 5.

[0078] Synthetic Material Substrate

[0079] 1^(st) layer 222 nm MgF₂

[0080] 2^(nd) layer 7 nm ZrO₂

[0081] 3^(rd) layer 256 nm MgF₂

[0082] 4^(th) layer 10 nm ZrO₂

[0083] 5^(th) layer 255 nm MgF₂

[0084] 6^(th) layer 13 nm ZrO₂

[0085] 7^(th) layer 245 nm MgF₂

[0086] 8^(th) layer 20.5 nm ZrO₂

[0087] 9^(th) layer 121 nm MgF₂

[0088] Air

[0089] The total thickness of the layer sequence is 1149.5 nm, wherein1099 nm is allotted to the MgF₂ layers, and 50.5 nm is allotted to theZrO₂ layers required for the additional antireflex effect.

[0090] However, this layer sequence, and the material MgF₂ having a lowrefractive index the employed, only show the substantial antireflexeffect since a hard layer of MgF₂ is generally not possible. However,the material having a high refractive index is quite allowed to vary andto be deposited on the outside in a more abrasiveproof coating having arespective refractive index. Then, in combination with SiO₂ as a hardlayer TiO₂ and ITO (indium tin oxide) are possible, e.g. for the UVrange, ZrO₂ in the VIS and NIR ranges, Ta₂O₅ apart from alreadymentioned HfO₂ as well.

[0091] The low total material thickness being below {fraction (1/20)} ofthe total thickness of the layer sequence is substantially then withrespect to the use of the material having a high refractive indexwherein it is assured that with manufacturing the coating a minimum heatload of the synthetic material substrate occurs. With the use of indiumtin oxide (ITO) the coating is allowed to have an additional antistaticeffect.

EXAMPLE 5

[0092] An example of application is to coat spectacle glasses andoptical windows as well made of translucent synthetic materials(polycarbonate or PMMA), e.g. for displays of measuring instruments, andin vehicles which should comprise an abrasion resistance according toISO 9211-02-04 (rubber test, 40 strokes having a force of 10 N).

[0093] Prior to the substantial layer deposition, the substrates to becoated are bombarded for 30 seconds in a coating plant with argon ionshaving an energy of appr. 100 eV, and a current density of appr. 0.1mA/cm².

[0094] If substrates of PMMA are concerned this preparatory treatment issubstituted with a Surface modification of poly(methyl methacrylate)which is described in detail in DE 197 03 538 A1 wherein reference toits disclosure is made in a full scope.

[0095] Then, to increase the adhesion performance and stability of thecoating a vaccum plasma treatment is performed prior to the depositionof the coating. During the plasma threatment oxygen and a gas containingwater is supplied. It should be preferably kept an equivalent fractionof water which corresponds to a relative air humidity of at least 40%Thereby, the substrate material will be ablated on the surface, and inparallel to this a chemical reaction is initiated in which the surfaceof the substrate will be changed under forming a polymer layer. Thepolymer layer formed on the surface of the substrate clearly differsfrom the untreated material with respect to its chemical composition,and therefore with its characteristics. This superficial polymermaterial comprises a particularly high fraction of methylene andhydroxyl groupes. With PMMA as a Substrate, the characteristic C—O andC═O groups are decomposed during this preparatory treatment modifyingthe surface.

[0096] For the manufacture of the coating layers of SiO₂ and Ta₂O₅ arealternately deposited wherein the growing layer is bombarded with Arions having an energy of 80 eV (SiO₂) and 120 eV (Ta₂O₅), and having acurrent density of appr. 0.1 mA/cm². By means of the deposition of thelayer system indicated in the example 1, the reflection of a coatedsurface in the visual spectral range of 420 nm to 670 nm is reduced to≦1%. The translucency for visual light (transmission) is increased from92% to >98% by means of a two sided coating. The coating methodincluding the treatment with argon ions is completely described in DE197 52 889 C1.

[0097] The coating proves successful in the abrasion test according toISO 9211-02-04 without any defect formation as well as in an abrasiontest performed with steel wool. Thus, the scratch resistance has beensubstantially improved with respect to that of the uncoated substrate.FIG. 6 shows the measured transmission with one sided coating of a layersystem manufactured in this manner.

EXAMPLE 6

[0098] A reflection reducing coating from an alternating change layersystem consisting of individual layers which are formed of SiO₂ andTa₂O₅ has been deposited on a Substrate of a cyclo olefin polymer or acyclo olefin copolymer as they are commercially available under thetrade names “Zeonex” and “Topas”. This coating altogether consists of 27individual layers, and thus a high mechanical stability can be achieved.

[0099] With the alternate formation of the individual layers in thevacuum, an ion aided method can be advantageously performed wherein therespective layer is bombarded with argon ions. With respect to an SiO₂layer it can be advantageously operated with an energy of 80 eV, andwith respect to a Ta₂O₅ layer it can be advantageously operated with anenergy of 120 eV, and a current density of appr. 0.1 mA/cm²,respectively.

[0100] Although very thin layers of Ta₂O₅ have been formed whichcomprise a high refractive index such a coating is insensitive tosystematic errors of layer thickness, and a higher proper meeting oflayer thicknesses will not be claimed.

[0101] The coating withstands the abrasion test according to ISO9211-02-04 without any formation of defects as well as an abrasion testwith steel wool. The scratch resistance is equivalent to a pure SiO₂layer having the same thickness.

[0102] A Substrate with such a coating has a very good climateresistance, and fissuring or the ablation of the coating could not beobserved at temperatures between −35° C. and +100° C. such that theclimate resistance is clearly higher than that of pure individual SiO₂layers or other well-known reflection reducing layers or layer systems.

[0103] With respect to this example, the following structure wasselected for the reflection reducing coating:

[0104] Cycloolefin Polymer or Cycloolefin Copolymer Substrate

[0105] 1^(st) layer 34 nm SiO₂

[0106] 2^(nd) layer 3 nm Ta₂O₅

[0107] 3^(rd) layer 236 nm SiO₂

[0108] 4^(th) layer 2 nm Ta₂O₅

[0109] 5^(th) layer 254 nm SiO₂

[0110] 6^(th) layer 2 nm Ta₂O₅

[0111] 7^(th) layer 255 nm SiO₂

[0112] 8^(th) layer 3 nm Ta₂O₅

[0113] 9^(th) layer 255 nm SiO₂

[0114] 10^(th) layer 3 nm Ta₂O₅

[0115] 11^(th) layer 254 nm SiO₂

[0116] 12^(th) layer 4 nm Ta₂O₅

[0117] 13^(th) layer 254 nm SiO₂

[0118] 14^(th) layer 4 nm Ta₂O₅

[0119] 15^(th) layer 253 nm SiO₂

[0120] 16^(th) layer 4 nm Ta₂O₅

[0121] 17^(th) layer 253 nm SiO₂

[0122] 18^(th) layer 4 nm Ta₂O₅

[0123] 19^(th) layer 254 nm SiO₂

[0124] 20^(th) layer 3 nm Ta₂O₅

[0125] 21^(th) layer 424 nm SiO₂

[0126] 22^(th) layer 5 nm Ta₂O₅

[0127] 23^(th) layer 225 nm SiO₂

[0128] 24^(th) layer 30 nm Ta₂O₅

[0129] 25^(th) layer 23 nm SiO₂

[0130] 26^(th) layer 56 nm Ta₂O₅

[0131] 27^(th) layer 92 nm SiO₂

[0132] Air

[0133] In FIG. 7 the reflection performance of a two sided coatedcycloolefin polymer having the trade name “Zeonex” is illustrated.

[0134] With the graphical representation it will be appreciated that thereflection within the visual spectral range of light that is withwavelengths between 420 nm to 670 nm can be uniformly kept below 0.5%.Simultaneously, the translucency for the visual light (transmission) canbe increased from 92% to >98% such that very good opticalcharacteristics are ensured.

[0135]FIG. 8 shows the structure of a layer system in a pictorialschematic.

1. A reflection reducing coating on a Substrate which is formed from an alternating change layer system of different layer materials each having a lower and higher refractive index, characterized in that the sum of the layer thicknesses of layers having a higher refractive index is ≦5% of the total layer thickness of the coating, and said layers of the material having a higher refractive index within the layer sequence of said change layer system are uniformly distributed.
 2. A reflection reducing coating according to claim 1, characterized in that said coating is deposited on an optically translucent substrate.
 3. A reflection reducing coating according to claim 1 or 2, characterized in that said substrate consists of a synthetic material or a temperature sensitive crystal material.
 4. A reflection reducing coating according to any one of the claims 1 to 3, characterized in that said layer material having a higher refractive index is selected from oxides or fluorides of the IVth and Vth class B elements.
 5. A reflection reducing coating according to claim 4, characterized in that said layer material for the layers having a higher refractive index are Ta₂O₅, ZrO₂, HfO₂, TiO₂ or indium tin oxide.
 6. A reflection reducing coating according to any one of the claims 1 to 5, characterized in that said SiO₂ is a layer material for said layers having a lower refractive index.
 7. A reflection reducing coating according to any one of the claims 1 to 5, characterized in that said MgF₂ is a layer material for the layers having a lower refractive index.
 8. A reflection reducing coating according to any one of the claims 1 to 7, characterized in that said layer forming the boundary layer towards the air is formed from a material having a lower refractive index.
 9. A reflection reducing coating according to any one of the claims 1 to 8, characterized in that layers of different materials having a higher refractive index are arranged in said layer system.
 10. A reflection reducing coating according to any one of the claims 1 to 9, characterized in that said layer system is matched to a predeterminable wavelegth range of light.
 11. A reflection reducing coating according to any one of the claims 1 to 10, characterized in that at least five individual layers form said layer system.
 12. A reflection reducing coating according to any one of the claims 1 to 11, characterized in that said coating has a total thickness between 500 and 2500 nm.
 13. A reflection reducing coating according to any one of the claims 1 to 12, characterized in that said substrate is an optical or opto-electronic element.
 14. A reflection reducing coating according to any one of the claims 1 to 12, characterized in that said substrate is a spectacle glass. 